Chlorotoxin is a peptide component of venom from the scorpion Leiurius quinquestriatus that has been shown to bind specifically to tumor cells. Chlorotoxin has been used as a targeting agent to deliver cytotoxic and/or imaging agents to a variety of tumors including metastatic tumors and brain tumors such as malignant glioma. For example, chlorotoxin has been conjugated to the radioactive isotope iodine-131, and such chlorotoxin conjugates have been shown to be effective anti-tumor therapeutic agents. Other chlorotoxin conjugates including protein fusions, such as a chlorotoxin-GST fusion protein attached to saporin, have also been shown to result in a significant and selective killing of tumor cells.